Okay, overcrowding changes the question dramatically. I didn't realize your sim was never making it in the door in the first place. Without mods to assist or adding extra schools, most schools will max out at around 25-30 students (and teachers depending on their start times) trying to get into a single door all at once with timeouts on the attempts to get inside and foot-stomping shows of frustration when sims collide with each other. Some schools/worlds may end up this way sooner than that. Under EA standard, only one door can be used on school buildings for entry/exit. This is also why it becomes important to arrive at class an hour or more early when in Uni World.
Here is one way to go about fixing that:
http://modthesims.info/d/568897/additional-entry-and-exit-routing-slots-to-reduce-door-crowding-schools-university.html
Or add additional schools (or do both, which is what I usually do now as the above mod is so brilliant). With both schools bulldozed at the same time and then added back in, that should force all sims to re-register but they will always do so at the one that is closest to their homes. Depending on where you have placed a second school relative to their housing, they may all end up re-registered at one again (either one). NRaas MasterController can be used to force a more divided registration, it's under Sim > Intermediate > Career > Choose School, but that won't last as sims age up and new generations of students arrive. StoryProgression can be used to prevent sims from constantly trying to re-register once you have placed them. StoryProgression (together with GoHere, which must also be present) can also be used by way of its Lot Options to restrict one school to children and the other to teens, or by gender, Net Worth, zodiac sign (to choose something random) or any other factor. That works out well too, but can be a bit fussy to arrange and sometimes takes a few tries before sims get the idea of which school they are supposed to be going to, so it takes some watching over. One of my worlds after a baby boom was so heavily populated that it had two elementary and three high schools in it, now down to one and two due to declining enrollment as the baby boom has aged upwards.
One can also use the NRaas Careers mod and its Schools module to add specialty (usually considered private, but they don't have to be) schools at various rabbitholes like Miltiary, Arts, Culinary, Science around town to spread things out more. There is also an option for Homeschooling. The specialty schools each focus on special skills in addition to regular child/teen development like the public schools do. Whether one wants to restrict those to students expected to perform better (or worse) academically, by traits, or wealth, etc. or open them up to all depends on how granular one wants to be about managing the entire town. For many of us this is a big part of the fun as I believe a better developed town provides more interesting friendship, co-worker, and dating pools for the future generations, but other players only wish to have their own sims progress properly and let the rest of the town pretty much do its own thing. :)
